Typically metalization schemes require barrier or adhesion films in order for a metal film to properly adhere to and make good contact resistance with underlying layers. A common metalization scheme for aluminum interconnects uses a combination of a titanium film followed by a titanium nitride film, over which, an aluminum or an aluminum alloy film is deposited. Aluminum suffers from several problems, such as its relative high resistance compared to copper and electromigration problems.
Attempts have been made to use tantalum and tantalum related compounds in forming interconnect structures. For example, either a pure tantalum film or a tantalum nitride film is used as a barrier/adhesion film for a copper interconnect. However, tantalum can be very difficult to remove using a polishing process, and tantalum nitride has adhesion problems with some types of copper films.
Other barrier/adhesion films could be used for the interconnect schemes. In one, a titanium film is deposited followed by a titanium nitride film that is followed by a titanium-rich titanium nitride film. Following the deposition of the titanium-rich titanium nitride film, an oxygen plasma is then used to convert that film to a titanium oxynitride compound. Unfortunately, titanium oxynitride can be too resistive. Although tantalum can be used in place of titanium in the oxynitride compound, the problems with the relatively high resistance is expected to continue to be a problem.
In another interconnecting scheme, an aluminum or aluminum alloy film can be capped with a titanium-rich titanium nitride compound followed by a stoichiometric titanium nitride compound. As previously discussed, copper does not adhere to titanium nitride very well. Even if tantalum is used to replace the titanium, the adhesion problem still exists with tantalum nitride.